1. Field of the Invention
The present invention relates to a vertically aligned liquid crystal display device and a method of manufacturing the same and, more particularly, a liquid crystal display device having a structure in which the alignment direction of liquid crystal molecules is split into plural directions in a pixel and a method of manufacturing the same.
2. Description of the Prior Art
The active matrix type liquid crystal display device can prevent the crosstalk by providing a switch, which is turned off at the time of non-selection to cut off the signal, to each pixel, and thus exhibits the excellent display characteristic rather than the simple matrix type liquid crystal display device. In particular, since the TFT (Thin Film Transistor) has the high driving capability in the liquid crystal display device which employs the TFTs as switches, such liquid crystal display device can exhibit the excellent display characteristic which is almost equivalent to the CRT (Cathode-Ray Tube).
FIG. 1 is a sectional view showing a structure of the normal TN (twisted nematic) liquid crystal display device.
The TN liquid crystal display device has a structure in which a liquid crystal 69 with positive dielectric anisotropy is sealed between two sheets of glass substrates 61, 71 which are arranged to oppose to each other. The TFTs (not shown), pixel electrodes 62, bus lines 63, a flatting layer 64, and an alignment film 66 are formed on an upper surface side of the glass substrate 61. The pixel electrodes 62 are formed of ITO (Indium-tin Oxide) as transparent conductive material. A voltage is supplied to these pixel electrodes 62 at a predetermined timing via the bus lines 63 and the TFTs to correspond to the image. The flatting layer 64 is formed of insulating material to cover the pixel electrodes 62 and the bus lines 63. A horizontal alignment film 66 formed of polyimide, etc. is formed on the flatting layer 64. An alignment process is applied to a surface of the alignment film 66 to decide the alignment direction of liquid crystal molecules when no voltage is applied. As the representative process of such alignment process, the rubbing process in which the surface of the alignment film is rubbed by a cloth roller along one direction is known.
While, a black matrix 72, color filters 73, a flatting layer 74, an opposing electrode 75, and an alignment film 76 are formed on a lower surface side of the glass substrate 71. The black matrix 72 is formed of metal such as Cr (chromium) such that the light does not transmit into areas between the pixels. The color filters 73 consist of three color filters of red (R), green (G), and blue (B). Any one of R•G•B color filters 73 opposes to one pixel electrode 62. The flatting layer 74 is formed to cover the black matrix 72 and the color filters 73. The opposing electrode 75 formed of ITO is formed under the flatting layer 74. The horizontal alignment film 76 is formed under the opposing electrode 75. A surface of the alignment film 76 is also subjected to the rubbing process. In this case, the rubbing direction of the alignment film 66 is different by 90° from the rubbing direction of the alignment film 76.
The glass substrates 61, 71 are arranged to put spherical or cylindrical spacers 79 between them. A layer thickness of the liquid crystal 69 (referred to as a “cell thickness” hereinafter) is kept constant by the spacers 79. The spacers 79 are formed of plastics or glass, for example.
In addition, polarizing plates (not shown) are stuck onto the lower surface side of the glass substrate 61 and the upper surface side of the glass substrate 71. In the normally white mode liquid crystal display device, two sheets of polarizing plates are arranged such that their polarization axes intersect orthogonally with each other. In the normally black mode liquid crystal display device, two sheets of polarizing plates are arranged such that their polarization axes are positioned in parallel with each other.
In this disclosure, the substrate on which the TFTs, the pixel electrodes, the alignment film, etc. are formed is referred to as a “TFT substrate”, and the substrate on which the color filters, the opposing electrode, the alignment film, etc. are formed is referred to as a “CF substrate”.
FIGS. 2A and 2B are schematic views showing an operation of the normally white mode TN liquid crystal display device. As shown in FIGS. 2A and 2B, in the normally white mode liquid crystal display device, two sheets of polarizing plates 67, 77 are arranged such that their polarization axes intersect orthogonally with each other. Since the liquid crystal 69 with the positive dielectric anisotropy and the horizontal alignment films 66, 76 are employed in the TN liquid crystal display device, the liquid crystal molecules 69a in the neighborhood of the alignment films 66, 76 are aligned in the rubbing direction of the alignment films 66, 76. As shown in FIG. 2A, in the TN liquid crystal display device in which two sheets of polarizing plates 67, 77 are arranged such that their polarization axes intersect orthogonally with each other, the liquid crystal molecules 69a sealed between two alignment films 66, 76 change their alignment direction helically as their positions come close from one substrate 61 side to the other substrate 71 side. At this time, the light which passes through the polarizing plate 67 enters into the layer of the liquid crystal 69 as the linearly polarized light. Since the liquid crystal molecules 69a are aligned to be gradually twisted, the polarization direction of the input light is also twisted gradually and thus the light can pass through the polarizing plate 77.
If the voltage applied between the pixel electrode 62 and the opposing electrode 75 is gradually increased, the liquid crystal molecules 69a start to rise along the direction of the electric field when the voltage exceeds a certain voltage (threshold value). When the sufficient voltage is applied, the liquid crystal molecules 69a are directed substantially vertically to the substrates 61, 71, as shown in FIG. 2B. At this time, the light which passes through the polarizing plate 67 fails to pass through the polarizing plate 77 since its polarization axis is not rotated by the layer of the liquid crystal 69.
In other words, in the TN liquid crystal display device, the direction of the liquid crystal molecules is changed from the almost parallel state to the substrates 61, 71 to the vertical state thereto in response to the applied voltage, and the transmittance of the light which is transmitted through the liquid crystal display device is also changed correspondingly. Thus, desired images can be displayed on the liquid crystal display device by controlling the transmittance of the light every pixel.
Meanwhile, the good viewing angle characteristic cannot be achieved by the TN liquid crystal display device having the above structure. That is, the good display quality can be achieved if the image is viewed from the vertical direction to the substrates, nevertheless the contrast is extremely lowered if the image is viewed from the oblique direction and also the density is inverted.
As the method of improving the viewing angle characteristic of the TN liquid crystal display device, the alignment partition is known. This can be attained by providing more than two areas which have different alignment directions in one pixel. More particularly, one pixel region is divided into two areas or more, and then their alignment films are rubbed along different rubbing directions respectively. Accordingly, since the light which is leaked from one area can be cut off in the other area, reduction in the contrast in the half tone display can be improved.
In recent years, the vertically aligned liquid crystal display device is watched with interest as the liquid crystal display device which is superior in the viewing angle characteristic and the display quality to the TN liquid crystal display device. The liquid crystal (positive liquid crystal) having the positive dielectric anisotropy and the horizontal alignment film are employed in combination in the TN liquid crystal display device, whereas the liquid crystal (negative liquid crystal) having the negative dielectric anisotropy and the vertical alignment film are employed in combination in the vertically aligned liquid crystal display device. In the vertically aligned liquid crystal display device, the liquid crystal molecules are aligned in the almost vertical direction to the substrates under the condition the voltage is not applied between the pixel electrode and the opposing electrode, while the alignment direction of the liquid crystal molecules is inclined gradually horizontally relative to the substrates when the voltage is applied between the pixel electrode and the opposing electrode.
By the way, in the vertically aligned liquid crystal display device in the prior art, like the TN liquid crystal display device shown in FIG. 1, the cell thickness is kept at a constant thickness by the spacers. The spacers are formed of spherical or cylindrical plastics or glass, as described above, and scattered onto any one substrate when the TFT substrate and the CF substrate are stuck together.
Therefore, in the vertically aligned liquid crystal display device in the prior art, the step of scattering the spacers is needed and the manufacturing steps become complicated. Also, the cell thickness becomes uneven due to variation in the scattering density of the spacers, and thus the display quality is degraded. In addition, if the vibration or the impact is applied to the liquid crystal display device, the spacers are moved to cause variation in the cell thickness. Furthermore, if the strong pressure is applied to the glass substrate, the spacers are sunk into the color filters, so that the cell thickness is also changed.